Bacteriophage P22 procapsids contain an internal scaffolding protein which leaves the structure when the DNA is packaged into the coat protein shell. The scaffolding protein co-assembles with the coat protein. Approximately 200 molecules of scaffolding protein assemble with about 400 molecules of coat protein to form the procapsid. Studies of the scaffolding protein showed that C-terminal fragments are competent to co-assemble with the coat properly. Gel studies suggested that there were more copies of shorter scaffolding proteins in these procapsids. Mass measurements with the STEM found that procapsids assembled with half-length scaffolding protein contained about twice the number of scaffolding molecules. This suggests that a shell-filling mechanism may be operating during procapsid assembly. Procapsids formed with scaffolding proteins of various lengths are being measured to test this.